In view of numerous studies showing the production and efficient repair of cyclobutyl dimers in normal cells, it has become vital to evaluate the production and reparability of other UV-induced products, which if inefficiently repaired, could play a large or possibly a major role in cellular UV sensitivity. Progress has been made in a number of areas. These include the development of HPLC and two-dimensional TLC methods for rapid and complete separation of the four thymine dimers and six stereoisomeric thymidine dimers. For the first time, the structure and stereochemistry of heterodimers of psoralen and thymine derivatives have been determined after isolation of sufficient quantities of these photoproducts. This information will further our understanding of photobiology of psoralen sensitization. Methods have been developed for the formation of epoxide of pyrimidines in both light and dark reactions. Pyrimidine epoxides have been considered as possible primary products of oxidation of nucleic-acid components, and their nucleophilic ring-opening reactions would offer an alternative mechanism for crosslinking of nucleic acids and other biomolecules. In addition, we are in the process of accumulating new non-dimeric photoproducts and adducts from UV-irradiated yet biologically active DNA samples so that their structures and stereochemistry can be elucidated.